1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic process or an electrostatic recording process such as a copying apparatus or a printer.
2. Related Background Art
In the image forming apparatus employing the electrophotographic process, the photosensitive member is generally charged by corona discharge.
In recent years, instead of such discharge method, there is being conceived an injection charging process practically involving no discharge.
In such injection charging process, it is required to achieve contact with the photosensitive member in a high density, but it is practically very difficult to maintain a roller or a blade in contact, without leaving a small gap, with the rotating photosensitive member.
It is therefore conceived to achieve injection charging by contacting a fine particle layer with the photosensitive member.
The Japanese Patent Application Laid-open No. 5-127490 proposes a charging apparatus of a magnetic brush type.
In the contact charging apparatus of magnetic brush type, conductive magnetic particles are held as a magnetic brush either directly on a magnet or on a sleeve incorporating a magnet, and the charging is initiated by contacting such magnetic brush in a stopped or rotating state with an image bearing body to be charged (hereinafter called photosensitive drum) and applying a voltage thereto. As the magnetic particles constituting the magnetic brush, the aforementioned Japanese Patent Application Laid-open No. 05-127490 describes that there are preferred magnetic particles with an average particle size not less than 30 xcexcm and not more than 70 xcexcm, free from protruding portions such as needle-shaped portions or edges, and made into a spherical shape in such a manner that the ratio of the longer axis and the shorter axis does not exceed 3 times.
However, in the image forming apparatus employing the contact charging apparatus of magnetic brush type as disclosed in the aforementioned Japanese Patent Application Laid-open No. 05-127490 as the charging means for the photosensitive drum and also employing the contact transfer charging process as the transfer means for a developer image from the photosensitive drum to a recording medium, there has been encountered a drawback of image defects such as image fog or ghost image, resulting presumably from defective charging.
Such situation occurs in case of paper-passing durability test utilizing, as the conductive magnetic particles for constituting the magnetic brush (hereinafter called carrier), xe2x80x9cspherical carrierxe2x80x9d particles formed by a polymerization process and having an average particle size of 50 xcexcm, a saturation magnetization of 200 emu/cm3 and a resistivity of 5xc3x97106 xcexa9cm.
However, satisfactory image formation, without the aforementioned image defects resulting from the defective charging, can be achieved with xe2x80x9ccrushed carrierxe2x80x9d formed by a crushing process and having an average particle size of 25 xcexcm, a saturation magnetization of 200 emu/cm3 and a resistivity of 5xc3x97106 xcexa9cm instead of the spherical carrier.
This is presumably because the finely powdered carrier particles formed at the crushing operation increase the contact area between the photosensitive drum and the carrier thereby increasing the charging ability to the photosensitive drum.
However, if the crushed carrier is used, the carrier may leak for example by adhesion of the fine carrier particles to the photosensitive drum as described in the aforementioned Japanese Patent Application Laid-open No. 05-127490 and such leaking carrier may be subjected to the pressure of the contact transfer charging apparatus in the transfer portion to cause damage to the photosensitive drum, thereby significantly deteriorating the service life thereof.
Also, the image forming apparatuses have been made more compact in recent years, but such compactization of the entire image forming apparatus is inevitably limited by the compactization of the individual means or device of the image forming process steps such as charging, exposure, development, transfer, fixing and cleaning.
Also the toner remaining on the image bearing body after the image transfer (residual developer) is collected as waste toner by cleaning means (cleaner), but such waste toner is preferably absent in consideration of the environmental protection.
For this reason, there is already commercialized the image forming apparatus of a xe2x80x9ccleanerless processxe2x80x9d (cleanerless system, toner recycling system) in which the cleaner is dispensed with and the residual toner remaining on the image bearing body after the transfer step is removed from the image bearing body by xe2x80x9ccleaning simultaneous with developmentxe2x80x9d of the development means (developing device, developing unit) and is collected and reused in the development means.
In such cleaning simultaneous with development, the toner remaining in a certain amount on the image bearing body after the image transfer is collected, at the development in one of the subsequent process cycles, by a fog-eliminating bias (a fog-eliminating potential difference Vback between a DC voltage applied to the development means and the surface potential of the image bearing body).
In such process, since the residual toner after the development is collected in the development means and is used again in one of the subsequent process cycles, it is rendered possible to eliminate the waste toner and to reduce the amount of cumbersome maintenance work. Also, the absence of the cleaner provides a large advantage in space, thereby enabling significant compactization of the image forming apparatus.
However, the use of such cleanerless system in combination with the magnetic brush charging process results in drawbacks of a pass-through ghost and a hue variation in case of a color image.
FIG. 26 shows an example of image defect by a pass-through ghost. FIG. 26 shows an A4-sized sheet on which a character xe2x80x9cAxe2x80x9d is formed in a position (1) (real image or main image). The character xe2x80x9cAxe2x80x9d in the above-mentioned position (1) is not completely transferred but partially remains on the photosenstive drum as residual toner, which should be collected in the developing portion but is not completely collected and is transferred again onto the recording material to form an image in a position (2), called pass-through ghost, that should not be present ideally. This explanation of the phenomenon is also supported by a fact that the distance a shown in FIG. 26 coincides with the peripheral length of the photosensitive drum used.
It will be understood that the aforementioned pass-through ghost phenomenon breaks the basic concept of the cleanerless system and results in the destruction of the system itself. In the following there will be explained why the cleanerless system is broken by the pass-through ghost phenomenon.
The investigation of the present inventors has revealed that, in the cleanerless system, the collection of the transfer residual toner in the developing portion is limited and that the efficiency of collection and the mass of the transfer residual toner per unit area are mutually correlated.
More specifically, in a chart shown in FIG. 27, the ordinate indicates the level of the image defect (pass-through ghost) as shown in FIG. 26, evaluated in five levels, while the abscissa indicates the amount of the transfer residual toner per unit area. The amount of the transfer residual toner is adjusted by regulating the transfer current, thereby varying the transfer efficiency. In FIG. 27, marks ◯ indicate the evaluated levels of the images in the position (1) in FIG. 26 while the marks x indicate the evaluated levels of the image in the position (2) (namely the levels of generation of the pass-through ghost images).
The marks x in FIG. 27 indicate that the pass-through ghost starts to be generated about where the amount of the transfer residual toner exceeds 0.06 mg/cm2, and becomes more severe as the amount of the transfer residual toner increases.
However, the comparison of the marks x with the marks ◯ evidently indicates that the level of the actual image remains quite acceptable even in the area where the pass-through ghost is generated. The level of the actual image becomes unacceptable only after the transfer efficiency is extremely lowered to a level where the amount of the transfer residual toner is equal to or larger than about 0.16 mg/cm2, but has a considerably wider margin for the amount of the transfer residual toner, in comparison with the marks x.
As will be apparent from the foregoing description, the cleanerless system is made unacceptable by the presence of the pass-through ghost even in a situation where the formed actual image is quite acceptable.
Also in the color image forming apparatus of tandem or other type, there is encountered a drawback that a toner image once transferred onto the recording material or the intermediate transfer member is transferred again to the photosensitive drum (hereinafter called re-transfer) at the transfer of another toner image of a next color, whereby the desired toner image cannot be obtained.
In a color image forming apparatus employing the aforementioned cleaning system simultaneous with the development (for example an image forming apparatus formed by combining a color image forming apparatus of tandem system and a cleanerless process), a mixture of the transfer residual toner and the re-transferred toner is collected by the fog-eliminating bias Vback at the development. Since the re-transferred toner is different in color from the transfer residual toner, the collection of the re-transferred toner together with the transfer residual toner at the development results in mixing of colors in the developer. With the repetition of the image forming cycles, the toner of different color is accumulated in the developing apparatus, so that the desired color can no longer be obtained. Such phenomenon is particularly conspicuous when the amount of the re-transferred toner is large. In case the contact transfer charging method is employed in such image forming apparatus, there is encountered a drawback that the color hue immediately shows fluctuation because of the aforementioned reasons, whereby the cleanerless system is broken.
Such phenomenon will be explained in more detail in the following. As shown in FIGS. 28A and 28B, there were employed two image forming stations, which are respectively called a first state positioned at the upstream side in the conveying direction of the recording material and a second station positioned at the downstream side. In each of the first and second image forming stations, there is only illustrated a photosensitive drum and image forming process means therefor are omitted. In each of the first and second image forming stations, there was formed a stripe image (in the main scanning direction) representing an area of 6% for the A4 size.
In the second image forming station, the re-transferred toner generated in the course of image formation is collected, upon reaching the developing portion of the second image forming station, in the developing apparatus by the fog-eliminating bias.
For the purpose of quantitative evaluation, the re-transfer rate xcex7rtr is defined, as shown in FIGS. 28A and 28B by:
xcex7rtr=b/(a+b)xc3x97100[%]
wherein a [g/cm2] is the toner amount per unit area of the recording material after the re-transfer, and b [g/cm2] is the amount per unit area of the toner re-transferred onto the photosensitive drum.
Also the transfer efficiency xcex7rtr is similarly defined by:
xcex7rtr=bxe2x80x2/(axe2x80x2+bxe2x80x2)xc3x97100[%]
wherein axe2x80x2 [g/cm2] is the toner amount per unit area of the recording material after the transfer, and bxe2x80x2 [g/cm2] is the amount per unit area of the transfer residual toner remaining on the photosensitive drum after the transfer.
This investigation employed yellow toner and magenta toner respectively in the first and second image forming stations and executed the formation of a magenta image (a stripe image in the main scanning direction representing 6% in A4 size), from an initial state (where yellow toner is completely absent in the magenta developing apparatus). The image formation was repeated in a similar manner for 10,000 sheets (intermittently by 100 sheets at a time), the color difference xcex94E between the initial image and the image after each 1000 sheets was measured with X-RIteSP68. The transfer efficiency of the yellow toner in the first image forming station was always maintained at 95%.
The result of the investigation is shown in FIG. 29, in which the abscissa indicates the number of passed sheets while the ordinate indicates the change in the color difference. A curve A indicates the result obtained with a contact transfer charging apparatus (transfer roller) in the second image forming station, while a curve B indicates the result obtained with a non-contact transfer charging apparatus (corona charger) in the second image forming station as a comparative reference. These configurations respectively correspond to FIGS. 28A and 28B.
In FIG. 29, the curve A indicates that the color difference increases as the number of the passed sheets increases, and exceeds an upper limit value 6.5 of the color difference, giving a same color in impression when about 5000 sheets are passed. On the other hand, the curve B shows a smaller change in the color difference, apparently reflecting the difference in the transfer charging method.
An object of the present invention is to provide an image forming apparatus capable of charging by conductive particles.
Another object of the present invention is to provide an image forming apparatus employing a magnetic brush charging system and a cleanerless system.
Still another object of the present invention is to provide an image forming apparatus comprising: an image bearing body; charging means for charging the image bearing body, the charging means having a charging particle in friction contact with the image bearing body and thereby charging the image bearing body; electrostatic image forming means for forming an electrostatic image on the image bearing body charged by the charging means; developing means for developing the electrostatic image on the image bearing body; and transfer means and for transferring a developed image onto a transfer material at a pressure contact portion where the transfer means is in pressure contact with the image bearing body; wherein a pressure of the transfer material to the image bearing body is not less than 1 g/cm and not more than 100 g/cm in line pressure.